The long-term objectives of the research are to develop our knowledge of how humans discriminate and recognize speech sounds and to apply this knowledge toward the explanation of specific phonetic regularities. The proposed studies will build on previous work in our laboratory to yield a more complete account of auditory factors in speech perception and of listener-oriented factors in speech production. In general, a phonological contrast is realized by a covarying set of phonetic properties, and this covariation is often quite uniform across languages. Typically, phoneticians have attempted to explain phonetic covariation in terms of putative physical constraints on speech production. Correspondingly, theorists within the field of speech perception have often assumed that phonetic trading relations (where the perception of one acoustic-phonetic dimension is affected by the setting of another acoustic parameter) result primarily from the listener's implicit knowledge of speech production constraints. An alternative hypothesis is that many significant types of phonetic covariations reflect a strategy of speech communities to enhance the auditory distinctiveness of phonological contrasts, and relatedly, that many phonetic trading relations are rooted in general auditory factors that are not specific to speech. To evaluate this alternative hypothesis, a variety of perceptual and auditory modeling experiments will be conducted, each designed to factor out the possible perceptual contribution of speech-relevant knowledge. The experiment will employ the following methods, among others: (1) Speech/nonspeech comparisons. Categorization performance on speech stimuli varying along certain acoustic dimensions are compared to performance on nonspeech stimuli that are acoustically analogous in certain relevant respects. (2) Cross-native-language comparisons. Certain phonetic dimensions are relevant in some languages but not others. It is possible to factor out the contribution of language-relevant experience by comparing categorization performance on such a phonetic dimension between a group of subjects whose native language exploits the dimension and a group of subjects for whom the dimension is linguistically irrelevant. (3) Auditory modeling. A realistic model of mammalian auditory-nerve response is used to predict performance on several types of speech perceptual task (e.g., discrimination, categorization and auditory distance judgements). These converging methods will yield important information about how the auditory representation of speech sounds constrains speech perceptual performance. The results will also permit a detailed evaluation of listener-oriented accounts of phonetic covariation. Finally, the research will provide a foundation for future work on how various forms of auditory pathology affect speech perception.